Process of treating hydrocarbons



March 1,

Gaps Inlet,

1932. F. E. FREY ET AL 1,347,238 Y PROCESS OF TREATING HYDROCARBONSFiled May 21, 1929 2 Sheets-sheet 1 5 Oufifiefi vlgvi. w 9 M 7 --r 1 v f,j I t l Exofihermzc Cracking Chamber & scrubbqr Extradim Flam?! Z l JBcnzol Guild affier exo kerm ic cracking Thermocouple g 157M317 Gas to6e 4/ undergo exothermic 2i cracking 5 .Colal diluent gas Z'q I confirolextent of cradrvng ZQW 2 RE. Frey -A- wycr d. 5 G.G.Obcrfcll z INVENTORS4 i f I ATTORNEY Patented Mar. 1, 19 32 "UNITED STATES PATENT oFFica 7FBEDERICIK E. FREY, JESSE A. GUYER, 4ND GEORGE G. OBERFEIL, OFBARTLESVILLE,

OKLAHOMA, A.SSIGNORS 'IO PHILLIPS PETROLEUM COMPANY, OE BARTLESVILLE,

GKLAHOMA IEROGESS or TREATING. nxnRocAnBoNs" Application filed May 21,1929. Serial No. 364,810.

The present invention relates to the cracking of hydrocarbons in thegaseous and/or Vapor phase to convert the same principally into aromaticcompounds typified by benaene,

toluene, and xylene.

It has been proposed to convert propane,

butane and the like, partially into crude benzol by cracking both in thepresence of and in the absence of catalysts. The prior art 10 workers,in an empirical way, have cracked ethane, propane and butane andmixtures thereof in a heated tube, and obtained light oils, gaseousproducts and a heavy tar.

' However, in investigations of this character, the chemical reactionsoccurring were not thoroughly understood, and insufli'cient importancewas attached to the time factor involved in the conversion of thehydrocarbons. We have made extensive investigations to study in asomewhat exhaustive manner the sequence of changes occurring during theI cracking of hydrocarbon gases, and particularly propane and butaneinto aromatic compounds and oils and tar, and to further de- 2 terminein a fairly quantitative degree the effect of temperature on the natureof these consecutive changes and-on the velocity with which they occur;Our. experiments show that at temperatures of 1250 to 17 50? F., andunder atmospheric pressure, the cracking of hydrocarbons, typified bypropane, butane andjmixtures of the same, proceeds endothermically withan increase in volume and theformation' of simple gaseous olefines to amaximum of 35 to 50% by volume, by familiar reactions in which theparaflin molecule splits into-two molecules, one an'olefine, and theother a simple paraffin 'or hydrogen. We have ascer- 40 taineddefinitely that in the case of propane or butane, the heat absorbed bythe endother mic reaction reaches a maximum of about 700 B. t. u. perpound'at approximately 1562.

F., at which state of maximum heat content,

the olefine content is approximately at the maximum. At approximatelythe tempera 'ture specified, this condition isattained in somewhat lessthan 0.002 minutes. Athigher cracking temperatures within the range(1250 to 17 F.) both olefine content and heat content attain a somewhathigher maximum value than at the lower ones, The time'- consumed by thisstage of reaction decreases rapidly with increase in the temperature atwhich it occurs.

The higher temperatures of cracking as beforestated give a highercontent of the olefines than the lower temperature for that time ofcracking, characteristic for each temperature, giving the maximum volumepercent of olefines. The chief reason for this is that the decompositionof ethane to ethylene and hydrogen C H C H +'H 37900 cal. is reversibleand rapid and endothermic at temperatures within our cracking range. Thehigher the temperature,'the more completely is'the ethane dissociated,at equilibrium. Ethane, ethylene and hydrogen are all formed in theearly cracking reactions,

"and, though influenced of course by other reactions in which ethyleneis destroyed, a hydrogenation equilibrium is nevertheless approached. Asa result, much of the ethane we find in the gases produced bytcracking.at a low cracking temperature, appears as ethylene and hydrogen at ahigh cracking temperature and augment the unsaturates content of thegas, and, since the dissociation is endothermic, the heat content ofvthe gas. The unsaturatesattains a maximum 10-15 percent higher at 1562F. cracking temperature, than at 1112 F.

Our experiments. indicate that. continued exposure to the crackingtemperature causes the olefines produced by the initial cracking toundergo an exothermic conversion or de-' composition into aromatic oilsand tar after a period of no less than tentimes as long as that requiredto e'flect the endothermic decomposition or conversion.

In endothermic cracking, it happens that the reaction in the usefultemperature range (1250-17 is so rapid that a practical coefficient ofheat transfer through confining surfaces as in a tube coil, develops atemperature at which the reaction-will just absorb the heat furnished.Furthermore, in the early stages of endothermic cracking, the velocityof cracking is greater than in the later stages, and a somewhat lowertemperature is therefore developed than in the later stages ofendothermic cracking. The time of cracking at single temperature,moreover, is more or less incidental to the economically rapidintroduction of the endothermic heat. In our experience, a tube coiloperating at full capacity permitted the introduction of all theendothermic heat with an exit temperature in the gas of 14:004425 F. Anexit temperature above l i50 F. showed the instability characteristic ofan incipient exothermic reaction. The value of 0.002 minutes at 1562 F.,is a maximum value, and an exposure time for exothermic cracking tentimes thatlong, 0.02 minutes, is a minimum value for developing amaximum amount of volatile oils. Unlike the endothermic cracking, thisvalue has been determined fairly accurately by us and is long enough tobe significant and indicate that a reaction chamher is desirable, andindicate the size to be used. A value for this ratio as high as 50 ispermissible, corresponding to an exothermic cracking time of 0.10minutes which gives a substantially optimum yield of volatile oils also,but how much greater the maximum value would be, since the time ofendothermic cracking given of 0.002 minutes is a maximum value we havenot determined.

' A maximum yield of benzene and toluene is thus developed, accompaniedby a somewhat less amount of tar, and very little carbon. During thisperiod, relatively little change occurs in the volume of the gas.However, of course, the olefine content of the gas decreasesprogressively. The heat evolved during this stage in cracking propane orbutane to a maximum yield of simple aromatics, is about 350 B. t. u. perpound, at a cracking temperature of 1552 F., at which temperature aperiod of 0.04 to 0.08 minutes is required.

We have discovered that the temperature is related to the duration ofthis exothermic cracking stage by the empirical formula T: 124.5 180 logt in which T=temperature in degrees Fahrenheit and t=time in minutes.

This formula applies for temperatures varying approximately between 1250to 1550 F. It is somewhat less accurate when utilizin highertemperatures up to'17 50 i ln regard tosaid formula, it may be statedthat experiments were conducted with the object of determining the roleof cracking time as well as temperature in producing an optimum yleld ofbenzol. A gas of the composition was submitted to cracking in anon-catalytic silica tube under conditions suitable for the A productionof benzol. The experiments were conducted at atmospheric pressure andthe carbon, tar, benzol and gases were separately measured. The effectof time and temperature on benzol yield is shown in the following table,in which time is expressed in minutes and benzol yield in gallons perthousand cubic feet of gas.

TabZe.Efi'ect of time of cracking on benzol yield at severaltemperatures.

Time Yield 1292" F.

At a iven temperature, the yield of benzolity without greatlyinfluencing the sequence of changes taking place'during cracking. Themaximum yield of benzol was roughly constant throughout the rather widetemperature range studied within the time range appropriate to andunique for each temperature.

The data of the table show the minimum time required to obtain asubstantially optimum yield of benzol at several temperatures withinarather widerange. The values are 1 approximately 0.5 minutes at1292 F.,0.2 minutes at 1382 F., 0.012 minutes at 1562 F., and somewhat less than0.003 minutes at 1742 These values may be used to determine atime-temperature relation which is expressed in a compact form by theequation .T= 1245 180 log t,

found to be required for forming a yield of benzol, it is obvious thatthe same timetemperature relation is applicableto the treatment ofgaseous olefines to form benzol and to all the simpler paraflins whichdecompose into gaseous paraffins and gaseous olefines. 'Methane, becauseof its great stability to heat is 'not converted into henzol under theabove mentioned time-temperature conditions.

This formula gives the minimum time of cracking to develop asubstantially optimum yield of light oil at any temperature within therange. Since the oil yield in the early stages of exothermic crackingincreases rapidly with prolonging of exposure, the for mula may,perhaps, give an oil-yield as much as 25 percent less than optimum, butit does define the more sensitive lower exposure time limit. An exposureof from 2 to 6 times this length will actually give an optimum light oilyield, and exposure times in the range 2 to 6 times the formula valuewould actuallybe used, the selection depending ,on gas-depletion, taryield permissible, etc., within the range 12501550 F. From 1550 to 1750F.

the high rate of self-heating and complications due to heat transfer,renderedour work less accurate, but we have actually obtained optimumyields at exposure times twice that calculated by the formula attemperatures up to 1922 F.; well above the range.

A several fold increase in the period of cracking above the shortestperiod above specified, which will givea virtually optimum yield ofvolatile oil causes relatively little change in yield. The oil producedat 1562. F. by the cracking period of .02 min-' utes given by theformula contains 20% .or,

so of unsaturates, chiefly butadiene and cy- "clopentadiene. Theremainder is benzene,

The oil produced .by'

toluene and xylene. a longer cracking period, say .10 minutes containsover benzene, and less than 3% unsaturated hydrocarbons; the optimumyield of volatile oil is substantially constant over the temperaturerange of. 1250 F. to

We propose to take advantage of the exotherinic stage of the crackingoperation and to control said'stage so as to crack hydrocarbon gases toform oils in a reaction chamber in which converslon occurs at atemperature advantageously hi her than that of the gas,

and this without t e addition of extraneous heat duringthe exothermicstage, this being.

possible by reason of a predominance of exothermic reaction. Broadlystated, our disf coveries indicate that the oil forming stage in thecracking operation takes place ,exothe'rmically within the approximateran 0 of 950 F. to 17 50* F., the range preferab y being between .1100F. and '1750 F; The

composition of the gas to be exothermically cracked or converted, mayvary greatly. We have ascertained that parafiins higher than methanewill absorb heat in cracking. Gaseous olefines, especially ethylene,have a posi tive heat of formation. In cracking to form oils of highercarbon content, methane, which has a high negative heat of formation, isformed in a relatively large amount, whet-her hydrogen is, or isnotpresent. These facts account for most of the exothermic effect. Themaximum exothermic effect will be obtained with a gas containing amaximum concentration of gaseous olefines and a minimum concentrationofparafiins higher than methane. The calculated temperature rise duringthe exothermic stage of the cracking of butane, for e'Xample,-.

at approximately 1562- F., to give a maximum yield of simple aromatics,is approximately 350 -F. A temperature rise, from this cause, ofover 200F. has been obtained in large scale operations, which will bespecifically set forth hereafter. If a rise in temperature of 50 F. beconsidered the minimum which will give a practical advantage, the weightpercent of gaseous olefines need not greatly exceed that of theparaflins higher than methane.

In this connection it maybe well at this time to direct attention to theaccompanying drawings, in which Figs. 1,2,3 and 4 are diagrammaticviews, partly in vertical section, ofsuitable apparatus for use inpracticing our improved processes.

Referring to Fig. 1, we have passed, for example, 8924 cubic feet perhour of av as consisting chiefly of butane through a. coil 5 .of threeand one-half inch tube, 200 feet long, into which heat was introduced byconvection of combustion gases in the cooler parts, and radiation in thehotter. As the I .rate offiring was increased, the temperature of thegas" leaving the tube coilincreased rapidly, until it reached 1300 F.After reaching this point, the temperature rose very slowly to 1400 F.while the rate of firing was greatly increased. The linear temperaturegradientin the tube coil was from 1300 at the beginning to 1400 F.at-the end of the last section of the coil, in which-section theheat-increment was lmown to be many times large enough to heat the gas100 F. Both I by the formula, while causing depletion of the theseeffects show heat absorption to be due sorption is such that 20 poundswill absorb 80 200=16,000 B. t. u. in the time of contact of the gas,which was about 0.005 minutes in the section of the tube coil wherecracking took place. The temperature would show almost no gradient frompoint to point of the cracking section if the reaction rate did notdecrease as the original constituents were destroyed, but since it does,a small temperature gradient is to. be expected, the equilibriumtemperature attained being somewhat higher near the end of theendothermic stage of cracking than at the beginning.

when enough heat was introduced to raise the temperature of the gasleaving the tube to above.1450 F., the temperature fluctuated, as wouldbe expected from the onset of the exothermic cracking, whichwouldmagnify fluctuations in gas rate or heat input. In our example, gasissuing at 1435 Efrem the tube coil* with a specific gravity of 0.94(air=l.00) and over 40 percent unsaturates was passed through aninsulated cracking chamber 6, of. 273 cubic feet capacity, in which thetemperature rose to 1520 F. and the gas left the chamber at 7 with aspecific gravity of 0.55 and a high content of benzol. This correspondsto a 0.20 minute duration of exothermic cracking at a mean effectivetemperature of 1500 F. which is six times as long as the minimum time of0.037 minutes for developing an optimum light oil yield. A much longerexposure than 0.20 minutes would result in destruction of light oil,but" we have found an exposure atthis temperature six fold greater thanthe minimum given cracked gas in heating value, produces a maximum yieldof light oil more free from gum forming unsaturates than with theminimum exposure time.

The converted gaseous mixture leaving the reaction chamber 6, may bepassed by suitable conduits 8 and 9, first through a scrubber 10, andafterwards through an extraction plant 11; the light. oils beingseparated from the gas in the latter, and being partially suitable formotor fuel purposes.

As indicated, our process of exothermically cracking hydrocarbons may beapplied to other hydrocarbons than the gaseous parafiins. For example,the gases formed in the pressure distillation of petroleum, containparaflins higher than methane, as well as gaseous olefines, and thisproduct suitable as the initial starting material. A certain amount ofendothermic cracking, somewhat less than that required for ethane,propane, and butane, is necessary before such gases are in a suitablecondition to undergo exothermic crackmg.

As an example of pressure still gases suitable for treatment by ourprocess, the following is typical:

CO2 and 1- Olefines 9.

Hydrogen 5.

Nitrogen "*"T'f Calorific value 1800 B. t. 11. per cubic foot. Thecalorific value indicates 50 percent by weight or so of paraflins higherthan methane.

The olefines are largely propylene and butylenc. This gas would requiresome endothermic cracking at about 1400 to initiate exothermic cracking.A propane-butane concen- ,trate of cracking still gases would be evenmore suitable for cracking to benzol. Such a material containing 5percent propylene, 25 percent propane, 35 percent butylenes, 35 percentbutane has been obtained by us from pressure still gases.

The initial starting material may furthermore be the gases formed invapor phase cracking, and these are particularly suitable since theycontain as high, in some cases, as 75% olefines. Such gases will crackexothermically after only such a pre-heating as is necessary to initiatea moderate rate of exothermic reaction. As an example of a gas producedby vapor phase cracking, it may contain 50 percent unsaturates, with acalorific value of 1800 B. t. u. With the cracking conditions used, sucha gas would contain less ethane than unsaturates, and would need littleif any endothermic cracking to initiate exothermic cracking.

Petroleum may be cracked at temperatures of 1200 to 1600 F. almostwholly toproduce gases containing a large proportion of gaseousolefines, and such gases will undergo, according to the presentinvention, exothermic cracking. I

From the above, it is seen that gases typified by ethane, propane andbutane, and mu;- tures of these gases, are fairly rapidlyendothermically cracked at temperatures somewhat above 1250 F. with aconsiderable absorption of heat and with the formation of It is to benoted that an increase in temperature 'infboth stages of the -crackingoperatlon,

that'is, theendothermio stage and the exothermic stage, decreases thetime consumed in each stage to crack and obtain the desired results.

Broadly, in accordance with our 1nvent1on,

we propose to conduct thecracking operation in.two stages, anendothermic stage, and. an exothermic stage, and this, under peculiarlysuitable circumstances for each stage; and

in general, theiheatrequired forthe endothermic stage, may be introducedthrough any confining surface. For example, the gas may be passedthrough the coil tube 5, through which heat is applied externally. Thetemperature obtained in difierentparts of the tube are, generallyspeaking, the result of a balance between the rate of endothermicreactionand the rate of heat introduction.

Our experiments indicate that a temperature range of 1150 to 1500embraces the more practical values. Owing to the high velocity of thereactions occuring, cracking may beconducted economically in large scaleoperations at a temperature between 1300 F. and

1450 F. with the gas at a pressure of only a little above atmospheric.

The temperature balance has been explained above. It may bementionedthat it can be .put moreclearly in mathematicalform, but this would notbe especially useful, since the-several controlling factors cannot allbe evaluated, as, for example, the

change of velocity of endothermic heat ab sorption with extent ofendothermic cracking, but the practical temperature range of 1300 F. to1450 F. or 15 0O F. for conducting the endothermic cracking nearatmospheric pressure is practicahle for an iron or alloy tube coilin'which the maximum practical coeflicient of heat transfer is'used.

The exothermic stage may be conducted in the chamber 6, through thewalls of which no heat is introduced, and in which the carbon whichforms during'the operation may deposit. 4

Regardingthe insulated chamber, we have used acylindrical'chamber of 273cubic feet capacity of a length of about four times the diameter. Theinsulation wassupplied by building the wall 18 inches thick 0 andSilocel insulation brick. The lining is of firebrick. It is encased in asteel shell and stands on one end. The gas, after endothermiccrackingwas introduced atthe bottom at a temperature near that at which itleftthe tube coil and was. discharged fromthe top after exothermic crackinghad taken -place during its passage. The direction of flowj from bottomto top is preferred to t e,reverse direction, because the small decreasein specific gravity during exothermic cracking would, in the latter.arrangement,,tend to cause circulating and consequently, discharge partof the 'larly desirable i gas undercracked and part overcracked. 1

sulation, heat developed by the reaction may be used to attain thishigher temperature without mixing additional heating gases Ourexperimentsindicate further that the which would lower the quality ofthe gases.

produced by the process, andthis is particuthe initial startingmaterials are such and the process is carried out soas to produce a fuelgas. In extreme cases, however it may be necessary to impart a helpfultemperature increment prior to the exothermic cracking stage. However,if this is necessary, and if heated gases are used for this purpose,only a small amount thereof is necessary to impart a helpful temperatureincrement, since little of-the added heat is absorbed by endothermicreaction.

Regarding the addition of heat prior to exothermic cracking in thechamber 6a see Fig. 2), the following is an example: If the raw gas weresimply heated to 1300 F. about 1300 B. t. u. per pound would berequired.

If products of combustion at nearly flame temperature were added by wayofpipe 12, to complete endothermic cracking, enough would need to beadded to impart ZOO-B. t. u. per pound of heat of reaction, whereuponexothermic reaction would set in and a gas be produced containing benzoland perhaps I 35 per cent inerts. This not only would reduce the qualityof the gas, but, since the presence of tar and carbon suspension wouldrule out good heat recovery in some cases, the sensible heat carried bythe inerts would need to be lost. If, however, the raw gaswere heated t60 F. and then all the endother- 15 mic heat introduced in-a tube coilwithin the range 13001400 F. no such heating; gas

. III

would need to be added and yet, the tube-coil would be exposed to atemperature'only 100 F. higher. If a checkered paSSageWQ 6ternatelyheated and-used for endothermic cracking, or'if the' gaswerecracked under superatmospheric pressure at a lower temperature by reasonof a longer time of residence in the tube: coil, or'if theendothrmically cracked gas'were conducted. through an interconnectingtube and cracking chamber, in which a loss'of IOU-400 F.- of sensibleheat was permitted, the gas supplied to the cracking chamber 6 or 6;;would be endothermical ly cracked, but toocool sometimes to undermiccracking. The gas produced would contain only a little inerts.

In the case pf gases such as those from' 10 vapor phase cracking,whichwould absorb a negligible amount of'endothermic heat, the

gas could be preheated to 1100 F. in the tube coil 50, at whichtemperature the life of the tube would beflonger and the. cost lower,than at higher temperatures. The gas'fcould then be heated to1400 F. byintroducing hot products of combustion through pipe 12' f (rapid mixingshould reduce heat absorption from the water gas reactions formingcarbon monoxide), and the gas could then undergo self-heating andexothermic cracking to henzol. is

We will now proceed to described the process broadly, and, thereafter,several variations thereof.

Gases containing a substantial amount, preferably over 15% ofhydrocarbons higher than methane, that is, hydrocarbons such as ethane,propane, and butane, are subjected, preferably in the tube coil .5 or5a, to sufiicient preheating accompanied 'by endothermic crackingtoinitiate the exothermic stage of cracking to produce the olefines,which is then the conditions previously described, until a maximum yieldof aromatic compounds and oils has developed. For example; butane may bepassed through the tube coil in which it is cracked, preferably within atemperature range of 1300 to 1400 F., and discharged at 1450 F. with aspecific gravity of 1.08 (air=1.00), and anhnsaturated content of overby yolume. This gaseous product may then be passed through insulated-reaction chamber at the exit of which temperature ranges of preferably14:50" to l650 F. are developed. At temperatures within this range, theoptimum yield of aromatic com= pounds, or oil's, is obtained.

'However, if it is desired, the process, by a slight variation, willproduce carbon and drogen as the substantial end products. This willoccur if the exothermic reaction is allowed to proceed far enough to atemperature above 1600 F., and that temperature is maintained longenough to accomplish the desired decompositionf The hydrogen containinggas produced in accordance with the above will contain methane.-However, this may be reduced by using in the exothermic reactionchamber a suitable catalyst, such as carbon. Equivalent means may beused to increase thehydrogen content, and thereby make the hydrogensuitable for hydrogenation.

allowed to proceed'in the chamberv 6, under' If hydrogen forhydrogenation is desired, the chamber 6 or 6a may be filled with coke toact as a catalyst for increasing the hydro,- gen content, but such meanscannot be used to increase it greatly. The gas is, however, suitable forconversion into a purer hydrogen by other means, such as decompositionwith steam.

The exothermic cracking chambers may be subjected to fluctuations intemperature, and 7 it is desirable to provide means for reducing thetemperature of fluctuations and inequalities during the crackingoperation. This may be accomplished by disposing-fire-brick or othersuitable heat absorbing material in the exothermic cracking chamber.

The use of a checker ,brick fillingfor the reaction chamber, asindicated at 14.- in Fig. 3, would not remove fluctuations intemperature of long period, due to changes in self-heatas flow rate ortemperature of incoming gases varied, but the brick will smooth outfluctuations of short period by absorbing heat when the temperature ofthe gases rises above the brick temperature, and give it off when thegas falls below. Fluctuations of longer period or temperature drifthowever might need to be controlled by introduction of cold diluentgases through the pipe 12 in Fig. 2.

The checker brick might be of further advantage in smoothing outinequalities of cracking in this, that the small pressure gradient itwould cause might assist in giving all portions of gas an equal time ofpassage through the chamber, as by reducing edd currents out of the mainpath of the gas. X vertical cylindrical cracking chamber filled withcrackerbrick could be used. It might, for example, have about the samecubic content of empty space as if not brickwork were used, but brickwould take up about 46% of space," making .the chamber somewhat larger.

Fluctuations in the rate of flow, the temperatures and composition ofthe gases enter-- 1 ing the chamber to undergo exothermic cracking, aremagnified in;the temperature attained in the exit end of the chamber 6,6a or (SbQbecause of the high rate of self-heating at the hightemperature developed. Since 5 small variations in extent of crackinggreatly effect the characteristics of the final product,

the extent of cracking must be confined within narrow limits. When therate of flow of the gas undergoing cracking'and heat input. precedingintroduction to the cracking chamber is varied, as a means ofmaintaining a constant extent of cracking owing to the inertia of thesystem, due chiefly to storage of heat, considerable fluctuation incracking re-'* sults, and excessive cracking occurs periodi cally.

A closer control may be obtained. by the introduction of a coolingdiluent in a small 7 amount through the pipe 12, as heretoforementioned, either in the gas before it enters I the exothermic reactiolichamber, or at some drop due to heat loss through the chamber walls. Thetemperature rise, due to selfheating, depends on the composition of .thegds entering the chamber, the temperature at which it enters, and thetemperature-time relation as it passes through the chamber.

For a given constant composition, temperature, and velocity of gasentering the reaction chamber, there would be a constant extentof.cranking maximum temperature attained if no heat were thrown back by theself-heated gas to'gas which had as yet .undergone less self-heating,but since there is, the heat thrown back canincreas'e the rate ofself-heating gradually even under these constant conditions of theentering gas. The

- resulting temperature climb would eventually result in overcracking.The rate of firing the tube, coil 5% in which the endothermic crackingtook place could he suddenly decreased to. arrest the temperature rise.But

if this were done, the temperature climb in the top oithe crackingchamber could still go on while the furnace reached the new lowertemperature equilibrium, and when the temperature rise did stop, if therate of heating had beenreduced a little too much,.in order to bringabout a quick response, the chamber temperature would drop, less heatwould be thrown back, less self-heating thentake place, and the chambertemperature would.

continue to drop. Heat loss through the chamber walls. could reduce thisfluctuation somewhat, but this would necessitate more heat introductionin the tube coil, which it is desirable to keep at a minimum. Theefl'ect ofmagnifying in outlet temperature small fluctuations in inlettemperatureisthen' to be expected.

The introduction of cold diluent gas through pipe 12to arresttemperature climb, does so,'both by reducingthe inlet gas temperature,and hence rate of initial self-heating, and by decreasing the time ofpassage,

permit the exothermic cracking temperature to drop off, so a slightaverage excess of heat would need to be introduced. The amount ofdiluent gas would need to be great enough. to prevent excessivetemperature rise, when the fluctuation in operating conditions causesmaximum values. With ideally constant conditions in the tube furnace,the correctdegree of exothermic heating would take place, and the amountof diluent gas would be vanishingly small. Presumably, with the firingot the tube furnace and the flow rate of raw gas under good control, theamount of diluent would neverneed to exceed 5% of the gas being cracked,and the amountwould vary between 5% and zero as fluctuations were oflsetwhen they appeared. 4

The temperature attained, the unsaturates content and specific gravityof the completely cracked gas, are all properties which vary greatlyenough with extent of cracking to perniittheir continuous determinationto be used as an index of extent of cracking and means of controllingtheintroduction of di-' luent gas. Temperature is a convenient lndex touse, and it could be'used either to regulate .the rate of continuousintroduction of diluent gas or percentage of time open of anopenand-shut valve device. For example, the introduction of the colddiluent gas may be advantageously controlled by use ofa thermocouple 15(Fig. 2) or the like, which utilizes conditions at the top of thecracking chamber 6a to control the'operation of valve means 16 whichregulates the admission ofthe cold gas.

Our experiments also indicate that the formation. of carbon proceedsrapidly in the exothermic cracking stage, if the cracking continuesafter the maximum yield of benzol has developed. The'high rate. ofcracking at the conclusion of the exothermicstage aggravates the troubledueto'carbon forma-,

tion. The extent of cracking may be better controlled, and carbondecomposition in the exit pipe 17 (Fig; 3) reduced by introducing atthepoint of exit from the chamber, enough cold gas to' reduce thetemperature to such a degree that further cracking is virtuallyarrested. .The introduction of cold-gas to arrest cracking would have tobe done between theexit 17 and the thermocouple 1'8 measuring themaximum temperature developed and used to control extent of cracking.The amount to be introduced through pipes 19, for instance, to reducethe temperaturefrom a reasonable value of 1550 ,F. to 1400 F., at whichlatter temperature cracking would be nearly arrested, would be about 10%byweight of the gas'cracked. The introduction will be contiguous and ata and hence time of self-heating. Both these Hired rate.

efl'ects take place at once on introduction of cold gas. The heatintroduced in the tube coil 5 a would need always to be so great that noSince the velocity of the exothermic reaction is greatly increased by arise of tem- 'perature, the accelerating of the temperature Ifluctuation in operating conditions would rise, especially in the earlystage oftheexothermic cracking, will permit a smaller size for thereaction chamber for a given amount of gas to be put therethrough. Thismay be accomplished by introducing the endothermically treated gas soasto provide turbulence,

thereby mixing the incoming gas with the gas which has already undergonesome selfheating. In the last stages of cracking, however, turbulence isundesirable, because a portion of the undesirable liquid unsaturates.and olefine gases will be discharged before their conversion occurs. Inthe latter stage, moreover, temperature instability becomes serious, andthe state of cracking must be controlled. This control may beaccomplished by providing an exothermic cracking chamber 60 whichconsists of two adjoining chambers 6dand 6e, communicating by one ormore constricted openings 6 f. The gas enters the lower chamber througha pipe 20, directed in such a manner that active turbulence is inducedin the chamber 6d to cause mixture of the incoming gas with gas that hasalready undergone some exothermic cracking. The gas then passes throughthe restricted opening (where cold gas is introduced through a pipe 21at an adjusted rate 'to maintain a constant extent of cracking),

into the adjoining chamber 66, through which it moves in an upwarddirection to an exit 22, at which cold gas may be introduced, if de-,sired, (as at 19 in Fig. 3) to arrest the crackmg.

Our experiments, in which hydrocarbons are cracked first endothermally,and then exothermally, while controlling the relationship between thetemperature-em'ployed in and the duration of each cracking stage toproduce an optimum yield of aromatic compounds, have been carried outunder atmospheric pressure or slightly above, but it is clear that theprocess may be carried out at higher pressures.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described, or portions thereof, butit is recognizedthat various modifications are possiblewithin the scope of the inventionclaimed.

What we claim and desire to secure by Letters Patent is:

l. The process of converting hydrocarbons, comprising thermally treatingraw hydrocarbon fluids to produce a gaseous mixture of aliphatichydrocarbons containing an olefine content sufficient to insuresubsequent exothermic cracking and having a temperature between 950 F.and 1500 F." suitable for exothermic conversion, then subjecting saidmixture without the addition of heat to exothermic cracking at atemperature between 1259 and 1750 for an interval of time substantiallyas expressed by the formula T=1245 180 log t, to convert the ole- -turebetween 1250 and 17 50 F.'for an interval of time substantially asexpressed by the formula T=1245180 log t, to convert the olefines intoheavier hydrocarbon oils, introducing a cold diluent gas into themixture during the exothermic reaction for control-- ling such reaction,and then separating said oils so produced.

3. The process of converting hydrocarbons, comprising thermally treatingraw hydrocarbon fluids to produce a gaseous mixture of aliphatichydrocarbons containing an olefine content sufficient. to insuresubsequent exothermic conversion and having a temperature between 950 F.and 1500 F. suitable for exothermic conversion, mixing a hot diluent gaswith said mixture to obtain said temperature, then subjecting thecombined mixturewithout the addition of heat to exothermic cracking at atemperature between 1250 and 1750 F., for an interval of time mixturewithout the addition of heat through an exothermic cracking zone at atemperature between 1250 and l750- F; for an interval of timesubstantially as expressed by the formula T=1245180log ,t, to converttheolefines into heavier hydrocarbon oils, introducing into the exit end ofsaid. zone a cold diluent gas for controlling said final temperature,and then separating said oils so produced.

' FREDERICK E. FREY;

JESSE A. GUYER.

GEORGE G. OBERFELL.

